Soft tissue remodeling is a biophysical phenomenon that occurs at cellular and molecular levels. Molecular contraction or partial denaturization of collagen involves the application of an energy source, which destabilizes the longitudinal axis of the molecule by cleaving heat labile bonds of a triple helix. As a result, stress is created to break the intermolecular bonds of the matrix. This is essentially an immediate extra-cellular process, whereas cellular contraction requires a lag period for the migration and multiplication of fibroblasts into a wound as provided by a wound healing sequence. In higher developed animal species, the wound healing response to an injury involves an initial inflammatory process that subsequently leads to the deposition of scar tissue.
The initial inflammatory response consists of the infiltration by white blood cells or leukocytes that dispose of cellular debris. Approximately seventy-two hours later, proliferation of fibroblasts occurs at the injured site. These cells differentiate into contractile myofibroblasts, which are the source of cellular soft tissue contraction. Following cellular soft tissue contraction, collagen is laid down as a static scar supporting matrix in the tightened soft tissue structure. The deposition and subsequent remodeling of this nascent scar matrix provides the means to alter the consistency and geometry of soft tissue for aesthetic purposes.
In light of the preceding discussion, there are a number of dermatological procedures that lend themselves to treatments which deliver thermal energy to skin and underlying tissue to cause a contraction of collagen and/or initiate a wound healing response. Such procedures include skin remodeling/resurfacing, wrinkle removal, and treatment of the sebaceous glands, hair follicles, adipose tissue, and spider veins.
Currently available technologies that deliver thermal energy to the skin and underlying tissue include electromagnetic energy, optical (laser), ultrasound and direct heating with a hot surface. In particular, electromagnetic energy may take the form of Radio Frequency (RF) energy.
RF based surgery demands knowledge of the electrical properties of the applied RF. These properties include accurate values of power, energy, and current delivered to a patient. For example, to receive the desired cosmetic effects using an RF energy delivery system a precise accumulated dose of RF energy must be delivered to the patient. Contemporary treatment protocols require accurate control of either RF current or RF power until a prescribed quantity of energy is accumulated or an appropriate time-out period has elapsed. Because of the differences from one patient to another, tissue resistance and RF current passing through the patient are highly variable. Consequently, to maximize the power to the patient it follows that a matching impedance of the electromagnetic energy source used in the RF surgery must be adaptable. Knowledge of the properties of the applied RF, patient tissue resistance, patient RF current and matching impedance are required to choose a current that will result in reaching a prescribed energy level within a specified treatment time period.
The high frequency RF signals of an electromagnetic energy delivery system, which may be in the Megahertz range, are delivered to the patient over electrical cables that are generally two meters long. The electromagnetic energy delivery system typically comprises an electromagnetic energy delivery device (typically a handpiece) to transmit the electromagnetic energy, and an electromagnetic energy source (generator) to produce the RF signals. RF energy is coupled into the patient capacitively through a tip in the handpiece.
A challenge with transferring the RF energy capacitively is that the patient has stray capacitance to earth ground that provides a bypass path for the RF current, leading to skewed measurements of the RF current or RF power. For example, taking measurements of the RF current or RF power at the generator (which may be remote from the patient) is challenging because cable impedances, insert capacitance, patient stray bypass capacitance, and the resultant effects on the measurement data value are all variable. However, the RF current and RF power must be known to calculate the total RF energy transmitted to the patient. Too much RF energy may result in burns, while too little RF energy may not provide the desired result.
To optimize the transfer of RF energy and maintain high electrical efficiency in the generator, the adaptable matching network may be utilized. The matching network compensates for capacitive and inductive reactance in the RF cables, insert capacitance, variable stray capacitance to earth ground, and variable tissue resistance. The matching network operates to maximize the RF energy transferred to the patient.
Tunable reactive components in the matching network may be adjusted to achieve an acceptable impedance match between the RF generator output stage and the compensated RF load impedance. The matching network is adjusted prior to a surgical RF delivery such that the RF current and voltage are approximately in phase and a directional coupler indicates a minimum in the reflected power coefficient, or ratio of reflected power to forward power.
Current sense transformers located at the RF output and return sense current delivered to the patient. However, a variable current that bypasses the patient through stray capacitance may be difficult to remotely determine. The bypass current may be needed to accurately calculate the amount of RF energy that has actually been transferred to the patient.
Calculating root mean square (RMS) values of the RF current and RF power at frequencies in the MHz range can also be challenging. Measuring the RMS RF current or RMS RF power at high frequencies is difficult due to the state of contemporary sensor devices. One way to measure RMS RF current and voltage is through an active peak detector. The RMS value is then calculated using the standard formula of 0.707 times the peak value. However, the active peak detector only detects and measures the high and low peaks of an RF signal without any consideration of possible harmonics. Additionally, the active peak detector is limited to applications where there is a pure sinusoidal waveform. Furthermore, the operating frequency for an active peak detection circuit requires that it detect peaks at many times the operating frequency of the signal it is measuring. Due to limitations in contemporary technology, this method is challenging and results in less accurate readings.
Another way to measure the peak value of the RF current and voltage at high frequencies is to utilize a diode circuit. However, diode circuits generally produce nonlinearities and harmonics or otherwise introduce inaccuracies into the measurements. At the operating frequencies of an electromagnetic energy delivery system, these inaccuracies make diode circuits a less than attractive alternative.
As shown in detail above, measuring the outputs of an RF surgical device presents numerous challenges. Consequently, there is need for an improved method and apparatus for performing critical RF signal measurements of an RF energy delivery system.